The mushroom species Agaricus bisporus (Lange) Imbach, also known as Agaricus brunnescens Peck, is a well known and widely cultivated commercial mushroom. At least one distinct variety of this species of mushroom has been the subject of a U.S. Plant Pat. No. 7,636, incorporated herein by reference.
Notably, a distinctive characteristic of Agaricus bisporus, that historically has defined the species, is that virtually all known strains predominantly produce only two spores on each basidium. Small percentages of aberrant basidia having as many as eight spores and as few as one spore have been shown to occur in various Agaricus bisporus strains. Nevertheless, following meiosis in a typical two-spored basidium of Agaricus bisporus, each spore receives two nuclei which are jointly necessary for fertility. As a result, most spores for this species of mushroom produce fertile, heterokaryotic progeny. This two-spored trait characterizes all commercially cultivated strains as well as the great majority of naturally occurring or "wild" strains of Agaricus bisporus thus far discovered. Such a trait of self-fertility poses a problem for the mushroom breeder because heterokaryons undergo little, if any, hybridization.
In contrast, all other known species of Agaricus produce predominantly four-spored basidia. In agaric fungi, the four-spored trait is associated with the production of mononucleate homokaryotic spores which are not fertile. In mushroom breeding, homokaryons, haploid strains which function in a manner similar to the gametes of plants and animals, are generally required for the practical crossbreeding of stocks to produce new hybrid strains. Homokaryons mate easily with other compatible homokaryons. However, although rare, it is possible to cross a homokaryon and a heterokayron or, in even rarer instances, to cross two heterokaryons.
To produce hybrids, homokaryons from the parent varieties of mushrooms must fuse and establish a common heterokaryotic cytoplasm. However, homokaryons are presently very difficult to obtain by conventional spore isolation from the two-spored Agaricus bisporus strains because less than three percent of the spores typically produced by such strains are homokaryotic. The great majority of spores of these strains produce fertile, heterokaryotic progeny as noted hereinabove.
Moreover, heterokaryotic and homokaryotic offspring are generally indistinguishable from one another except by genetic screening, such as by the use of allozyme or DNA markers, which is time consuming and costly. Homokaryons are also difficult to obtain by other presently available methods. For a more complete description of some conventional methodologies for the recovery of Agaricus bisporus homokaryons, and some difficulties and drawbacks thereof, see "Strategies For The Efficient Recovery of Agaricus bisporus Homokaryons" by Kerrigan et al. in Mycologia, 84(4), 575-579 (1992), hereby incorporated by reference.
Therefore, the need exists for a process which will permit the mushroom breeder to obtain large percentages and fractions of homokaryons relatively quickly, efficiently and inexpensively from the breeding stock of the Agaricus bisporus mushroom.